Page 312 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

If the vibrations are not controlled, the object may even get totally destroyed. A good example of this phenomenon
is the shattering of window glass panes due to the vibration caused by low flying aircraft.
Construction: Mechanical resonance type frequency meters consist of an eletromagnet and a set of metallic
reeds arranged in front of the electromagnet. The frequency meter is connected across the supply like a voltmeter,
taking care about the voltage rating (Fig 2) .

Fig 1 & 2

Fig 3 shows the shape of the reed and these reeds are of about 4mm wide and 0.5 mm thick. One end of the reed
is fitted on a base, and the other overhanging end carries a white painted surface as the indicator and sometimes
referred to as flag.
The reeds are arranged in a row and the natural frequency of the reeds differs by 1/2 cycle. This 1/2 cycle
difference is possible between the reeds due to the difference in the weights of the reeds. The reeds are
arranged in an ascending order (Fig 4a), and generally the natural frequency of the centre reed is the same as
that of the supply frequency (50Hz).
Working: When the frequency meter is connected to the supply, the electromagnet produces a magnetic field
which alternates at the rate of the supply frequency. The reed, which has its natural frequency coincident with that
of the alternating magnetic field, vibrates more than the adjacent reeds Fig 4(b).
The flag of this vibrating reed makes it possible to note the frequency of the supply from the scale marking of the
frequency meter. Though the other reeds also vibrate, Fig 4(b), their magnitude will be much less than the reed
whose natural frequency is exactly in coincidence with the supply frequency.
Fig 3 & 4

Advantages and disadvantages
The reed type frequency meter has the following advantages.
The indications are independent of i) the wave form of the applied voltage and ii) magnitude of the applied
voltage, provided that the voltage is not too low. At a low voltage the flag indication of the reed will not be reliable.
The disadvantages are the meter cannot read closer than half the cycle frequency difference between adjacent
reeds and the accuracy greatly depends upon the proper tuning of the reeds.

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CITS : Power - Electrician & Wireman - Lesson 50-53 CITS : Power - Electrician & Wireman - Lesson 50-53